Surface pressure (pi)-area (A), surface potential (DeltaV)-area (A), and dipole moment (mu (perpendicular to))-area (A) isotherms for N(1,1-dihydroperfluorododecyl)-N,N,N-trimethylammonium chloride (C12-TAC) and perfluorododecanoic acid (FC12) on the substrate of 0.01 M sodium chloride at pH 2.0 were investigated at the air-water interface by the Langmuir method and the ionizing electrode method. The temperature dependence of the transition pressure of each component was not larger than that of normal hydrogenated surfactant. The apparent molar entropy, enthalpy, and internal energy changes of phase transition from the disordered to the ordered state were calculated. Surface potentials (AV) were analyzed using the three-layer model proposed by R. J. Demchak and T. Fort (J. Colloid Interface Sci. 46, 191-202, 1974) for FC12 and Gouy-Chapman treatment for C12-TAC. The contributions of the omega -CF3 group and the head group of the vertical component to the dipole moment, mu (perpendicular to), were estimated. The new finding was that the pi -A curves are shifted to an area smaller than a molecular area of two pure components for the mole fraction (x) of perfluorododecanoic acid of x greater than or equal to0.3. The molecular areas negatively deviate from the additivity rule at discrete surface pressures. Assuming a regular surface mixture, the Joos equation (Joos, P, and R. A. Demel, Biochim. Biophys. Acta 183, 447, 1969) for analysis of the collapse pressure of mixed monolayer allowed estimation of the interaction parameter; xi = -4.20 at x less than or equal to0.5 and xi = -0.24 at x>0.5 between two fluorinated amphiphiles.